1. Field of the Invention
The present invention relates generally to a confocal microscope, and more particularly to an optical microscope that has high resolving power and a great focal depth and is capable of providing real time observation of images.
2. Discussion of Related Art
A confocal microscope comprising a confocal scanner designed to scan a sample through an objective using illumination light passing through a pinhole substrate having a plurality of pinholes while the pinhole substrate is rotated has so far been known in the art. For instance, a typical confocal microscope is disclosed at great length in U.S. Pat. Nos. 3,926,500 and 4,927,254.
An increase in the degree of integration of ICs recently developed in the semiconductor field now leads to IC samples and mask samples that cannot be identified with the resolving power of a conventional optical microscope. An inspection of such samples may be made using electron microscopes, interatomic force microscopes, etc. However, such microscopes are not suitable for total inspection machines on mass-production lines because much time is taken for inspection, samples break down, etc. A confocal type laser scanning microscope having a higher resolving power than does a conventional microscope, on the other hand, is not perfect for use on mass-production lines in view of inspection time, etc.
A confocal microscope comprising a pinhole substrate, to which the present invention is applied, now attracts attention in the semiconductor field because it has characteristic features that the aforesaid microscopes do not have. That is, the confocal microscope has a confocal effect resulting from a specific arrangement where an array of pinholes are located at a position conjugate with a sample surface, and so it is possible to achieve resolving power and contrast higher than would be possible with a conventional optical microscope. In particular, the resolving power in the optical axis (Z) direction is so high that sample information in the height direction can be obtained with high precision. Also, the confocal microscope enables a sample to be observed in real time or at a speed higher than video rates with human eyes or on an image pickup device as in the case of an optical microscope, because a scan of the sample by pinholes takes place by the high-speed rotation of the pinholes lying within a microscope field. In addition, the confocal microscope can be manipulated using a high-intensity white light source with no need of using a laser, etc., so that chromatic information, too, can be observed with ease. By reason of such characteristic features that are different from those of a laser scanning type confocal microscope, much attention is paid to the confocal microscope.
A specific requirement for an IC sample is, on the other hand, that sample information in a step be concurrently observed at a great focal depth because the IC sample has a stereoscopic structure. With the confocal microscope, however, it is impossible to meet the aforesaid requirement because its depthwise resolving power is high whereas its focal depth is short.
The simplest and most ordinary method of achieving a focal depth increase is to restrict the NA (numerical aperture) of illumination light using an aperture stop. With this method, however, there is a lowering of the resolving power in a lateral direction perpendicular to the optical axis. Some proposals have now been put forward to achieve a focal depth increase without causing any lowering of the lateral resolving power. For instance, "Progress in Optics", II, p. 131, North-Holland Pub. Co. (1963) introduces a technique for achieving a multifocal lens by disposing a zonal form of phase film having a phase shift X at a pupil position of a phototaking lens. JP-A 7-63995 proposes to achieve a focal depth increase without any lowering of resolving power by means of a filter through which a substantial portion of NA passes slightly, i.e., without recourse to the simple restriction of the NA of illumination light. A method using image processing, too, is proposed in the art. For instance, JP-A 55-140805 puts forward a method for removing the blurring of an image by image processing while a relative distance between an objective and an optical axis of a sample is varied, thereby achieving a focal depth increase without any lowering of resolving power.
Methods of observing a sample having a stereoscopic structure by making use of chromatic aberrations produced at a lens, too, have been put foward in the art. For example, JP-A 2-53016 proposes an objective system designed to obtain a multifocal image by the insertion of a chromatic aberration-producing lens system therein, so that a focal position can be varied per varying wavelength. JP-A 5-26635 proposes a confocal laser scanning type microscope wherein an optical system having chromatic aberrations and a semiconductor laser with a wavelength varying dependently on temperature changes are used to perform scanning in an optical axis direction in place of conventional mechanical scanning. JP-A 8-211296 proposes a confocal microscope system with a detector located at a position conjugate with a light source, wherein the overall system is so allowed to have chromatic aberrations that a focal position can be varied per varying wavelength to obtain a multifocal image thereby achieving a focal depth increase.
Conventional confocal microscopes are more improved in terms of resolving power than general optical microscopes inclusive of a confocal microscope comprising a pinhole substrate, to which the present invention is applied. To observe the shape of a sample in an optical axis direction, however, it is required to vary a relative distance between an objective and the sample. In other words, it is difficult to observe the stereoscopic structure of the sample in real time. The confocal laser scanning type microscope of JP-A 5-26635, too, has the same problem as the aforesaid confocal microscopes because scanning is performed in the optical axis direction by making use of chromatic aberrations.
Even with the system of JP-A 55-140805, it is again difficult to make a real time observation of a sample because the scanning in the optical axis direction is required although the system is not a confocal system. In addition, this system is inferior to a confocal system in terms of resolving power.
The systems described in "Progress in Optics", II, p. 131, North-Holland Pub. Co. (1963) and JP-A 7-63995 achieve high resolving powers and great focal depths by the modulation of illumination light or a pupil. Strictly speaking, however, it is impossible to achieve high resolving power although some considerable focal depth may be achieved.
JP-A 2-53016 is directed to an invention for the achievement of a multiple focus by making use of chromatic aberrations. When it is intended to increase the focal depth by simple use of this, however, blurred images of different colors are superposed one upon another, resulting in resolving power and contrast decreases. Even according to the teachings given in JP-A 2-53016, it is thus impossible to increase the resolving power and focal depth.
The confocal microscope of JP-A 8-211296 achieves high resolving power and a great focal depth because of making use of chromatic aberrations and a confocal optical system. A problem with this is, however, that the arrangement becomes complicated because of using a plurality of light sources. Also, since the confocal optical system (e.g., the objective) produces chromatic aberrations, some limitation is placed on using the confocal microscope as a microscope capable of ordinary observations. In addition, it is impossible to provide a confocal observation of a sample, so rendering it difficult to obtain chromatic information from the sample.
As mentioned above, with microscopes proposed so far in the art it is not possible to achieve resolving power and focal depth comparable to those of a confocal microscope. Even if this is possible, some problems would arise such as a complicated microscope structure, incapability of ordinary observations, and a failure in obtaining chromatic information from a sample.